1. Field of the Invention
The present invention relates to a plasma display panel having a magnesium oxide film that covers electrodes. More specifically, the present invention relates to improvement on the magnesium oxide film.
2. Description of the Related Art
In general, an AC type plasma display panel includes a magnesium oxide (MgO) film as a sputtering resistant film for protecting a dielectric layer that covers display electrodes. The magnesium oxide film is formed on the dielectric layer and is exposed in a gas discharge space. Since the magnesium oxide is a high gamma material that is apt to emit secondary electrons, the magnesium oxide film also contributes to reduction of a discharge starting voltage.
Many researches about the magnesium oxide film have been carried out for purposes of improving sputtering resistant property or improving discharge characteristics. For example, researches about crystal orientation and composition are described in Japanese Unexamined Patent Publication No. 10-106441 (Patent Document 1), Japanese Unexamined Patent Publication No. 11-135023 (Patent Document 2), Japanese Patent No. 3247632 (Patent Document 3), and Japanese Patent No. 3425063 (Patent Document 4). Patent Document 1 proposes replacing a magnesium oxide film having the (111) plane orientation with a magnesium oxide film having the (110) plane orientation, which has higher denseness than the (111) plane orientation, so that the sputtering resistant property is enhanced. Patent Document 2 describes forming a magnesium oxide film having the (110) plane orientation by a plasma CVD method. Patent Document 3 discloses a magnesium oxide film containing an element having a valence of three and an ionic radius that is similar to that of magnesium (e.g., silicon) is useful for reducing addressing errors causing no address discharge. In Patent Document 3, the reason why addressing errors can be reduced is understood to be that the contained element as impurities increases emission of secondary electrons. Patent Document 4 discloses a magnesium oxide film of (n00) plane orientation or (mm0) plane orientation (n and m are integers larger than or equal to one) in which six coordination impurity ion (selected from a group consisting of Fe, Ni, Co, V, Mn, Cr, Ru, Ti, Ta, Pd, Al, Rh, Sb and Nb) similar to magnesium is doped for enabling emission of more secondary electrons.
In addition, Japanese Unexamined Patent Publication No. 2006-28005 (Patent Document 5) describes improvement on film quality in which attention is focused on oxygen deficiency in a magnesium oxide film. This document specifies a desirable oxygen deficiency amount for reducing temperature dependence of a response time at a temperature within a range of −15 to 90 degrees centigrade, which is a value within a range of 5.0×1015 to 2.0×1017 per cubic centimeter. Here, the oxygen deficiency amount described in this document is an amount that is calculated from a total number of F center and F+ center measured by the electron spin resonance (ESR) method, and the response time is a period from application of voltage that causes a discharge to a time point when near infrared rays emitted by the discharge is undetectable (an end of light emission).
A decrease of response speed is known as a variation with the passage of time of a plasma display panel. That is, as an accumulative time of a display increases in a plasma display panel, a delay of discharge with respect to application of a voltage pulse for generating the discharge becomes conspicuous. The response speed is an indicator of a degree of the delay of discharge. This variation with the passage of time of a plasma display panel is possibly related to a certain change of the magnesium oxide film, but a cause of the change is not determined clearly.
Voltage waveforms for driving a plasma display panel incorporate the above-mentioned decrease of response speed when the plasma display panel is designed. If there is no or little decrease of response speed, a pulse width of the voltage pulse can be a minimum value adapted to an initial response speed or a value close to the minimum value. In reality, however, the response speed will decrease by approximately 30% after accumulative time of use of 20,000 hours, for example. Therefore, the pulse width is selected to be a longer value corresponding to an estimated decrease of response speed so that a discharge can be generated normally even if the response speed is decreased.
It is desired to decrease the pulse width particularly from a viewpoint of increasing an addressing speed. If a pulse width of an address pulse for generating an address discharge can be shorter than that in the current situation, the number of address pulses that can be applied in a limited period can be increased. Then, it becomes possible to display with higher resolution having more display lines. Alternatively, it is possible to enhance a luminance of display by increasing the number of display discharges corresponding to the decrease in time period necessary for addressing. In order to decrease the pulse width, it is necessary to improve the variation with the passage of time of the response speed.